Integrated circuits (ICs) are fabricated on wafers. Commonly, these wafers are semiconductor materials, such as silicon, and singulated to form individual dies. Through efforts of research and development, the size of the transistors making up the ICs has decreased to 45 nm and will soon decrease to 32 nm. As transistor size decreases, the supply voltage to the transistors decreases. The supply voltage is conventionally smaller than wall voltages available in most countries or battery voltages used in portable devices. For example, an IC may operate at 1.25 Volts whereas the wall voltage is 120V or 240V. In a portable device, such as cellular phone, the battery voltage may range from 6V at full charge to 3V at near empty charge.
A semiconductor IC may be coupled to a voltage regulator that converts available voltages at wall outlets or batteries to lower voltages used by the IC. The voltage regulator ensures a constant voltage supply is provided to the IC. This is an important function, because the ability of transistors to tolerate voltages under or over the target voltage is small. Only tenths of a volt lower may create erratic results in the IC; only tenths of a volt higher may damage the IC.
ICs are mounted on a packaging substrate, and the packaging substrate is mounted on a printed circuit board (PCB) approximately 1-2 mm thick during assembly. Conventionally, the voltage regulator is located on the PCB with the IC to which the voltage regulator supplies voltage. Placing the voltage regulator on the PCB separate from the IC results in a voltage drop between the voltage regulator and the IC that the voltage regulator supplies. For example, at a supply voltage of 1.125 Volts, a voltage drop of 0.100V may occur between the voltage regulator and the IC as the voltage passes through the PCB, packaging substrate, and IC. As the supply voltage decreases with shrinking transistor size, the voltage drop becomes a significant fraction of the supply voltage. Additionally, placing the voltage regulator on the PCB uses pins on the IC for the IC to communicate with the voltage regulator. The IC may send commands to the voltage regulator such as sleep or wake-up for scaling up or scaling down the voltage supply. The additional pins consume space on the IC that could otherwise be eliminated.
Reducing the voltage drop from the voltage regulator to the IC improves performance of the IC. Maximum frequency of a IC scales proportionally with supply voltage. For example, eliminating a voltage drop of 0.100V may increase a maximum frequency (fmax) of the IC by 100 MHz. Alternatively, if the voltage drop is reduced and maximum frequency not increased, power consumption in the IC is reduced. Power consumption is proportional to capacitance multiplied by a square of the supply voltage. Thus, reducing the supply voltage may result in significant power savings.
Further, conventional voltage regulators have slow response times due to the distance between the voltage regulator and the IC. In the event the current transients are too fast for the voltage regulator to respond, decoupling capacitors provide additional power to the IC. Voltage regulators located on the PCB often have response times in the microsecond range. Thus, large decoupling capacitors are placed on the packaging substrate to compensate for slow response times. The large decoupling capacitors occupy a large area. One conventional arrangement includes a bulk capacitor of microFarads and a multi-layer chip capacitor (MLCC) having hundreds of nanoFarads along with the voltage regulator on the PCB. The combination of the bulk capacitor and the MLCC supplies voltage to the IC while the voltage regulator responds to the current transient.
Attempts have been made to place voltage regulators on the ICs. However, voltage regulators include passive components such as inductors and capacitors that are also embedded in the ICs. Passive devices consume a large amount of IC area, which increases manufacturing cost. For example, a IC manufactured using 45 nm technology has a capacitance density of 10 femtoFarads/μm2. At this density a suitable amount of capacitance may consume over 2.5 mm2. Providing inductance to the voltage regulator conventionally uses an on-IC inductor or a discrete inductor mounted on the packaging substrate. In addition to consuming large areas on a IC, conventional on-IC inductors have a low quality factor.
The quality factor, defined by the energy stored in a passive component versus energy dissipated in the passive component, for a passive component embedded in a IC is low. Conventionally, the passive components are manufactured thin to fit in the IC and suffer conductive or magnetic losses that degrade the quality factor.
Additional problems arise when supplying voltage to the ICs of a stacked IC using conventional solutions. Specifically, supplying voltage to the second tier of a stacked IC is conventionally accomplished with wire-bonding. Wire-bonding is completed after assembly of the stacked IC and has a limited connection density based on size of the wire bond. Another conventional solution includes providing the supply voltage to the second tier IC with through silicon vias in the first tier IC. Through silicon vias have a high resistance resulting in voltage drop that further decreases performance and occupy space on the ICs that could otherwise be used active circuitry.
Thus, there is a need for a supplying voltage to a stacked IC that is in close proximity to circuitry of the stacked IC.